1. Field of the Invention
The present invention relates to an image forming apparatus and, in particular, relates to an image forming apparatus including a print head configured to discharge ink droplets upward from nozzle heads to form an image on a recording medium and to perform idle discharge to clean the nozzle heads.
2. Description of the Related Art
An inkjet recording apparatus employing a print head including nozzles and configured to discharge ink droplets from the nozzles and form an image on a recording medium is one type of widely used image forming apparatus. In such an image forming apparatus, idle discharge of the ink droplets is performed to maintain an optimal ink discharge status. This idle discharge is performed in such a manner that, after the print head is cleaned, ink droplets are discharged from the nozzles of the print head to an area outside the normal printing range either during or after printing. Accordingly, the idle discharge may also be referred to as dummy discharge or blank discharge.
In recent models of inkjet recording apparatuses, in general, several tens of idle discharges are shot at each nozzle every several seconds. Upon completion of the cleaning, the idle discharge is performed to prevent color mixing. Alternatively, the idle discharge is performed at the start of printing or regularly during printing. Several thousands to several tens of thousands of blanks are shot from each nozzle.
The idle-discharge ink is collected and recovered in an idly-discharged-ink collection tank. FIG. 13 is a cross-sectional view illustrating an example of a conventional collection tank for the idle discharge. The idle discharge tank 10 includes a porous sheet member 12 configured to receive ink droplets 13 from a print head 15 and a tank 11 to hold the collected ink. The ink droplets 13 idly-discharged from the print head 15 contact and are absorbed by the sheet member 12. Ink 14 leaking through the sheet member 12 falls and is collected in the tank 11.
Certain image forming apparatuses including a print head are configured to discharge ink droplets upward. Typically, a print head configured to discharge ink upward is adopted for an image forming apparatus capable of simultaneously forming images on both sides of the recording medium. In this type of image forming apparatus, a recording medium is conveyed between one print head configured to discharge ink droplets upward and a second print head configured to discharge ink droplets downward, so that images are formed simultaneously on both sides of the recording medium by the two print heads.
The idle-discharge ink discharged from the print head that discharges the ink droplets downward is collected in the idle discharge tank 10. On the other hand, the idle-discharge ink discharged from the print head that discharges ink droplets upward is collected in an idle discharge tank disposed above the print head. FIG. 14 is a cross-sectional view illustrating an example of a tank for the idle discharge corresponding to the print head that discharges the ink droplets upward as disclosed in JP-2001-341321-A.
The idle discharge tank 20 disclosed in the above publication includes a tank 21 above the print head 26. The tank 21 includes a protruding rib 23 disposed on a periphery of an opening 22 and preventing the upwardly discharged ink droplets from dripping off toward outside. In the idle discharge tank 20, the ink discharged as idle discharge from the print head 26 strikes an upper surface 24 of the tank member 21, flows down along the wall and is reserved in a bottom 25.
Further, there is a type of idle discharge tank that includes a function to prevent generation of mist (or fine ink droplets) from the ink droplets discharged from the print head. FIG. 15 is a cross-sectional view illustrating an example of an idle discharge tank capable of preventing generation of the mist as disclosed in Japanese Patent No. 3707274. This idle discharge tank 30 includes an ink absorbing sheet 33 disposed at an upper wall 32 of a tank member 31. The ink-absorbing sheet 33 absorbs ink droplets discharged from a print head 26 to an opening 34 of the tank member 31, thereby preventing generation of the mist.
However, in the conventional image forming apparatus, the ink droplets and the mist circulate inside the tank member and disperse from the opening due to an air stream generated by the idle-discharge ink droplets inside the tank member. Then, the ink droplets that cannot be held by the ink absorbing sheet disperse and contaminate the print head.